A subsea umbilical typically comprises a group of one or more types of elongated functional elements such as electrical cables, optical fibre cables, and optionally conduits for fluid transportation of for example, gas, water, or chemical products such as methanol. The functional elements can be assembled together in a helical or S/Z manner, and over-sheathed/over-armoured for mechanical strength and ballast. It is commonly desired for a single umbilical to be able to contain as many functional elements as are required for a particular application, for example as are required for a particular oil field where the umbilical is intended for use. Umbilicals are typically used for transmitting power, signals and fluids (for example fluid injection, hydraulic power, gas release, etc.) to and from a subsea installation.
Due to the development of ultra deep water (UDW) fields, subsea apparatus and equipment such as flexible risers, flow lines, umbilicals and pumps must be operated in more complex and harsh environments, including operating at higher temperatures and higher pressures, whilst also dealing with the conditions in ultra deep water. Such apparatus and equipment now requires to have improved performance and improved safety requirements than currently employed.
Some subsea production apparatus and equipment is located at the end of tie-backs, and requires the supply of various chemicals to be injected into the well. Such chemicals are commonly conveyed from the production facility on a surface vessel to the subsea equipment by one or more chemical injection tubes or lines, which tubes or lines are commonly structural elements within a subsea umbilical. However, with the development of longer UDW tie-back lengths, the associated pressure drop along the tube or line is also increasing, so that the subsea umbilical structure needs to be modified in response. Several parameters need to be considered in any modification of the umbilical structure, such as the tie-back length (and therefore expected pressure drop), the chemical flow rate, the injection pressure, and the pump discharge capacity available at the production facility.
The first possibility to overcome the above problems is simply to increase the size of the umbilical, in particular make it larger and more complex with a greater outer diameter. Increased umbilical performance should be possible to reach longer tie-backs with increased umbilical dimensions.
However, larger cross-section umbilicals also have a number of problems. Of these, the first is that there will be increased complexity in their manufacture, which may require greater machine capability as well as more complex umbilical termination connections and connection processes. Secondly, there is clearly an increase in costs, not only in CAPEX, but in qualification tests. Thirdly, there will be increased installation CAPEX and OPEX costs, (because of the increased dimensions and weight of the umbilical, as well as the increased installation equipment capability required).
A second possibility to overcome the above problems is to separate the chemical injection operation from the other functions provided or performed by the umbilical. In particular, to eliminate the chemical lines from the umbilical structure, such that the remaining umbilical is smaller and less complex. A development of this is to provide a subsea station with storage tanks for the chemical prior to injection. The tanks can be located close to the subsea processing equipment injecting the chemicals, but will require a local pump to assist pressure.
One example of this is shown in US 2012/0175125A, which shows a subsea oil and gas production Christmas tree control system with a HPU module as part of its pod, able to increase the local pressure for hydraulic or chemical fluids, thereby avoiding the need for long high pressure umbilicals.
However, the HPU is still required as a dedicated module in the pod, i.e. within the subsea production architecture, and is also externally exposed to the subsea conditions and environment. And the arrangement of a separate chemical injection operation requires two separate installation and laying operations, as well as organising the supply of a subsea station close to each subsea processing equipment, which again increases the CAPEX and OPEX requirements.